A particular turbojet engine for subsonic flight propulsion is disclosed in DE-OS 3720318 in which the shroud of the engine, which surrounds the turbine, has an inlet lip which is capable of being deformed mechanically by means of pairs of flaps which are distributed over the periphery. In this way, the shroud of the engine is said to be capable of being configured with a relatively slender aspect which is optimum in terms of fuel consumption and which is usually used for normal flight operation; nevertheless, critical conditions exist in the engine, e.g. during the starting phase (standing on the ground with the engine at full power) at which there is pronounced turbulent flow at the internal wall of the shroud at the lip (causing deficient and aerodynamically disturbed supply of air to the engine) and these conditions are said to be capable of being satisfied. In addition to an appropriate localized thickening of the walls of the inlet lip, the aforesaid pairs of flaps are said to provide channels through which outside air is forcibly blown in or out. The arrangement of flaps in the disclosed embodiment requires comparatively high construction and manufacturing costs. In addition to a peripheral arrangement of baffles, which is required, the flaps lead to a considerable increase in weight. An automatic static differential pressure control of the pairs of flaps is provided in the disclosed embodiment which has the disadvantage, among others, of giving rise to response characteristics which are relatively sluggish, in time, during critical stages of operation, whereby, for example, compression pumping as a result of the critical state may not be capable of being stopped in a timely manner or, as the case may be, sufficiently spontaneously.
Jet propulsion engines which are exclusively provided for subsonic flight operation with shrouded front fans or front propellers (prop fans) are also known (U.S. Pat. No. 3,931,708) in which reverse thrust or, as the case may be, a thrust reversal operation is produced, together with an appropriate negative positioning of the fan blades, to supply the fan and engine with intake air from the outside via the normally extremely thin-walled end of the shroud to be compressed to produce reverse thrust by a secondary mass flow towards the lip at the front or leading edge of the shroud. The trailing or rear edge of the shroud, normally the outlet end during forward thrust is provided with flaps which can be pivoted around axes of rotation which extend transversely with respect to the axis of the engine whereby normally the flaps form a trailing edge of the shroud, which is aerodynamically relatively slender with a gently convergent/divergent nozzle contour in the fully retracted position (in the case of normal horizontal flight); as regards the thrust reversal operation, the flaps are extended in a "spoiler-like" manner in order to provide a bell-shaped contour and hence to provide a supply of intake air to the fan via the local nozzle-like trailing edge of the shroud which is as free as possible of air turbulence from the shroud and which is uniform over its periphery.